Method for regulating and/or controlling a welding current source with a resonance circuit

ABSTRACT

The invention describes a method of controlling a welding current source ( 2 ) having a resonant circuit ( 27 ), whereby a bridge circuit ( 28 ) is controlled by a control system ( 4 ). A consumer, in particular a welding process, is supplied with energy via the bridge circuit ( 28 ) from a power source ( 29 ). In order to control the individual switching elements ( 32-35 ) of the bridge circuit ( 28 ), fixed pre-set control states S 1  to S 6  are stored and, during normal operation, the bridge circuit ( 28 ) is controlled on the basis of the control states S 1  to S 4  one after the other. If a change in resistance occurs at the consumer, the control system ( 4 ) switches to a special operating mode at the resonance frequency of the resonant circuit ( 27 ) and the bridge circuit ( 28 ) is controlled on the basis of the stored control states in order to run the special operating mode.

CROSS REFERENCE TO RELATED APPLICATIONS

Applicant claim priority under 35 U.S.C. §119 of Austrian ApplicationNo. A 88/2000 filed Jan. 20, 2000. Applicant also claims priority under35 U.S.C. 365 of PCT/AT01/00014 filed Jan. 19, 2001. The internationalapplication under PCT article 21(2) was not published in English.

The invention relates to a method of regulating and/or controlling awelding current source with a resonant circuit, of the type described inclaims 1 and 25.

An arc welding apparatus with a resonant circuit is already known frompatent document DE 44 11 227 A1. It consists of a phase-inverting sourcewith a rectifier, supplied by a mains voltage, an intermediate circuit,a rectifier co-operating with a clocked, primary-side currenttransformer and a secondary-side current transformer, with which awelding process, in particular a welding torch, is connected. Thecurrent transformer is clocked via a bridge circuit, in particular ahalf-bridge, the bridge circuit being made up switching elements. Theswitching elements of the half-bridge are electrically conductive duringa pre-set switching period. The switching elements of the bridge circuitare controlled and/or regulated so that the switching elements areswitched whenever either the resonant current or the resonant voltagedrops to zero and, to deactivate the switching elements, either theresonant current or the resonant voltage is damped to zero, and thisvalue is maintained for a brief period before deactivating and shuttingdown the switching elements.

The disadvantage of this approach is that a resonant circuit of thistype operates on a quasi or semi-resonant basis only, i.e. current isable to flow at the inductance in one direction only and voltage can beapplied to the capacitor in one polarity only.

The underlying objective of the invention is to propose a method ofregulating and/or controlling a welding current source with a resonantcircuit, whereby the welding current source is controlled and/orregulated depending on the output conditions of the consumer.

In a method of controlling a welding current source having a resonantcircuit in the form of a serial/parallel converter, in which a bridgecircuit comprising individual switching elements is controlled by acontrol system and a welding process is supplied with voltage andcurrent pulse from a power source via the bridge circuit, the controlsystem controle the bridge circuit is switched by the control statesstore in the control system so that the bias point on a characteristiccurve of the resonant circuit lies outside a resonance frequency and,whenever a change in resistance occurs in the welding process, thecontrol system operates with the bridge circuit in a special operatingmode, in which the bridge circuit is operated at the natural frequencyof the resonant circuit on the basic of the control states and sequencesstored for the special operating mode, this objective is achieved by theinvention as a result of the following features:

-   -   the control states fixed for normal operation of the bridge        circuit provided as a full bridge are a positive drive phase, a        positive freewheeling phase, a negative drive phase, and a        negative freewheeling phase and in the special operating mode,        the bridge circuit is switched from a drive phase, preferably        consecutively, into one of several alternative control states in        which the switching elements of the other bridge branch remain        activated, and the control system monitors how often a switch is        made from one special mode to the other special mode.

The advantage of this approach is that controlling the welding currentsource in this way in particular the bridge circuit, ensures that thebias point is always maintained on the same side of the resonance curve,in particular on the rising or falling edge of the resonance curve.Another advantage resides in the fact that, because of the differentoperating modes, in particular normal mode, special operating modeand/or the special control method, the resonant circuit continues torespond independently of an external power supply, thereby enabling theswitching frequency of the switching elements to be tuned and readjustedto the resonance frequency of the resonant circuit. Another significantadvantage is the fact that by using a method of this type to regulate awelding current source with a resonant circuit, a corresponding outputcurve can be obtained at which, with a low flow of current, acorresponding high output voltage is available for maintaining the arc,or igniting the arc, whilst allowing the power component and the weldingcurrent source to be kept to compact dimensions since the additionalenergy needed is made available by the resonant circuit.

The advantage of this approach is that controlling the welding currentsource in this way in particular the bridge circuit, ensures that thebias point is always maintained on the same side of the resonance curve,in particular on the rising or falling edge of the resonance curve.Another advantage resides in the fact that, because of the differentoperating modes, in particular normal mode, special operating modeand/or the special control method, the resonant circuit continues torespond independently of an external power supply, thereby enabling theswitching frequency of the switching elements to be tuned and readjustedto the resonance frequency of the resonant circuit. Another significantadvantage is the fact that by using a method of this type to regulate awelding current source with a resonant circuit, a corresponding outputcurve can be obtained at which, with a low flow of current, acorresponding high output voltage is available for maintaining the arc,or igniting the arc, whilst allowing the power component and the weldingcurrent source to be kept to compact dimensions since the additionalenergy needed is made available by the resonant circuit.

It is of advantage to store a sequence of control states of theswitching elements in the control system because a standard controlsequence can be set up for normal operation. Consequently a reproductionpulsed operation can be maintained at the consumer under constantconditions and hence a good welding result obtained.

Following a change in the resistance of the welding process; the prescheduled normal state, tuned to the resonant circuit, can be restoredif the control states are activated during a constant state in theconditions at the welding process on the basis of a time differencebetween two immediately consecutive current zeros of a status variableof the resonant circuit.

The objective may also be achieved by the invention as a result of thefeatures defined in the claims. The advantage of a welding currentsource of this type is that, because of the layout of the resonantcircuit and the fact that the different control states of the switchingelements and their switching duration are based on the resonancefrequency and a status variable in the resonant circuit and its zerocrossing is changed, requiring few pre-defined control sequences whichcan be selected during different operating states from the consumer, notonly is the system able to meet requirements with very little in the wayof additional hardware, a control can also be applied at high speed,which places less stress on the high-power switching elements used,thereby ensuring a longer service life. Another advantage to be had fromthe layout of the resonant circuit is that components with aconsiderably lower rating can be used to supply the consumer withsignificantly higher voltages, which means that use can advantageouslybe made of the time when a short circuit is being resolved, for exampletransferring a drop of melt from the electrode to the workpiece.

The invention will be described in more detail with reference toembodiments illustrated in the drawings.

Of these:

FIG. 1 is a schematic diagram of a welding machine and a weldingapparatus;

FIG. 2 is simplified, schematic illustration, showing a circuit diagramof a welding current source with a resonant circuit;

FIG. 3 is a simplified schematic illustration, showing a sequencediagram for the welding current source;

FIG. 4 is a simplified schematic illustration, showing a diagram of theresonance curve of the welding current source;

FIG. 5 is a simplified schematic illustration, showing a sequencediagram for controlling the welding current source when the resonancefrequency is constant;

FIG. 6 is a simplified schematic illustration, showing a sequencediagram for controlling the welding current source in the event of anincrease in resonance frequency;

FIG. 7 is a simplified schematic illustration, showing a sequencediagram for controlling the welding current source in the event of adecrease in the resonance frequency;

FIG. 8 is a simplified schematic illustration, showing another sequencediagram for the welding current source;

FIG. 9 is a simplified schematic illustration, showing a sequencediagram for controlling the welding current source in the event of anincrease in resonance frequency—as illustrated in FIG. 8;

FIG. 10 is a simplified schematic illustration, showing an output curveof the welding current source;

FIG. 11 is a simplified schematic illustration, showing a block diagramof the circuit for regulating or controlling the welding current source;

FIG. 12 is a simplified schematic illustration, showing another circuitdiagram for a welding current source with a resonant circuit.

Firstly, it should be pointed out that the same parts described in thedifferent embodiments are denoted by the same reference numbers and thepositions chosen for the purposes of the description can be transposedin terms of meaning to a new position when another position is beingdescribed.

FIG. 1 illustrates a welding system and a welding apparatus 1 for awhole range of welding processes, e.g. MIG-MAG welding and TIG weldingor electrode welding processes. Clearly, the solution proposed by theinvention may be used with a current source and a welding currentsource.

The welding apparatus 1 has a welding current source 2 with a powercomponent 3, a control system 4 and a switching element 5 co-operatingwith the power component 3 and control system 4. The switching element 5and the control system 4 are connected to a control valve 6 incorporatedin a supply line 7 for a gas 8, in particular an inert gas such as CO₂,helium or argon and such like, running between a gas storage 9 and awelding torch 10.

Furthermore, a wire feed device 11 such as commonly used for MIG-MAGwelding may also be activated via the control system 4 in order to feeda welding wire 13 from a supply reel 14 through a supply line 12 intothe region of the welding torch 10. Clearly, the wire feed device 11could also be integrated in the welding device 1, in particular in thebasic housing, in a manner known from the prior art, rather than used asan add-on device as illustrated in FIG. 1.

The current needed to strike an arc 15 between the welding wire 13 and aworkpiece 16 is fed via a supply line 17 from the power component 3 ofthe current source 2 to the welding torch 10 and the welding wire 13,the workpiece 16 to be welded also being connected to the weldingapparatus 1, in particular to the current source 2, via another supplyline 18 so that a current circuit can be established across the arc 15.

In order to cool the welding torch 10, the welding torch 10 can beconnected via a cooling circuit 19, with an integrated flow indicator20, to a fluid container, in particular a water container 21, so thatthe cooling circuit 19, in particular a fluid pump used to pump theliquid contained in the water container 21, can be activated when thewelding torch 10 is switched on, thereby enabling the welding torch 10and the welding wire 13 to be cooled.

The welding apparatus 1 also has an input and/or output device 22, bymeans of which a whole range of settings can be entered for weldingparameters and operating modes of the welding apparatus 1. The weldingparameters entered at the input and/or output device 22 are thenforwarded to the control system 4, from where they are applied to theindividual components of the welding system and the welding apparatus 1.

In the embodiment illustrated as an example here, the welding torch 10is also connected to the welding apparatus 1 and the welding system bymeans of a hose pack 23. The individual lines from the welding apparatus1 to the welding torch 10 are disposed in the hose pack 23. The hosepack 23 is connected by means of a connector device 24, known from theprior art, to the welding torch 10, whilst the individual lines in thehose pack 23 are connected to the individual contacts of the weldingapparatus 1 by means of connecting sockets and plug connectors. Torelieve tension on the hose pack 23, the hose pack 23 is connected via atension-relieving device 25 to a housing 26, in particular the basichousing of the welding apparatus 1.

FIGS. 2 to 7 illustrate an application of the welding current source 2with a resonant circuit 27, in particular with a serial/parallelconverter, FIG. 2 showing a simplified equivalent network of the weldingcurrent source 2. FIG. 3 is a sequence diagram illustrating how a bridgecircuit 28 of the welding current source 2 is controlled. A schematicillustration showing a frequency curve for the resonant circuit 27 isgiven in FIG. 4. FIGS. 5 to 7 show characteristic curves for controllingand/or regulating the welding current source 2 with the resonant circuit27.

Within the structure of the welding current source 2—illustrated in FIG.2—a power source is schematically indicated by 29. This power source 29is connected to a power supply network, in particular a public mainsnetwork, such as a 230 V or 400 V alternating current network, notillustrated. The alternating current supplied to the power source 29 isconverted into a direct current, although it would also be possible toconnect a boost chopper or buck chopper downstream, for example.

The power source 29 is connected via lines 30, 31 to the bridge circuit28 and supplies it with a direct current. The bridge circuit 28 may by afull bridge or of a half-bridge design, the particular embodimentillustrated here having a full bridge, comprising for switching elements32 to 35 and co-operating freewheeling diodes 36 to 39. The switchingelements 32 and 33 are provided in the form of what are known as IGBTtransistors and the switching elements as MOSFET transistors, forexample.

For control purposes, the individual switching elements 32 to 35 areconnected via control lines 40 to 43, shown by dotted-dashed lines, tothe control system 4, so that the switching elements 32 to 35 can beactivated and deactivated when power is applied to the control lines 40to 43. Connected at the mid-point of the bridge circuit 28 is theresonant circuit 27, in particular the serial/parallel converter,comprising an inductor 44 and a capacitor 45 connected in seriestherewith as well as another capacitor 46 connected in parallel with theconsumer. In the embodiment illustrated as an example here, the resonantcircuit 27 is indicated by broken lines.

A measuring device 47 is provided at the output of the resonant circuit27 for detecting the current and the voltage in the resonant circuit 27,the measuring device 47 being connected via lines 48, 49 to the controlsystem 4 for transmitting the current level and/or the voltage level.Connected to the measuring device 47 is a rectifier 50, which isillustrated by individual diodes, the output of the rectifier 50 beingconnected to output terminals 51 and 52 of the welding apparatus 1. Theconsumer, in particular the welding torch 10, is connected to theseoutput terminals 51 and 52 via the supply lines 17, 18, the weldingtorch 10 being illustrated as an equivalent electric wiring diagram inthe form of an ohmic resistance 53 and a line inductance 54 of thesupply lines 17 and 18.

The operating principle of the power source 29, the bridge circuit 28and the resonant circuit 27, in other words the welding current source2, will not be described in more detail from an electrical point of viewbecause such an arrangement is already known from the prior art. Whatwill be described is the control method and/regulating method used tosupply the consumer, in particular the welding torch 10, with currentand voltage in order to perform a welding process.

In principle, it should be pointed out that, as illustrated in FIG. 4,when using a resonant circuit 27, in particular when using aserial/parallel converter, the latter is always operated above or belowthe resonance frequency, preferably above the resonance frequency. Theresonance frequency is set depending on the output mode, in particularthe load resistance at the output terminals 51 and 52, in other wordsthe consumer, i.e. the resonance frequency changes whenever there is achange in resistance at the output, in other words at the welding torch10, due to the occurrence of a short circuit for example, in which casea corresponding frequency band can be set on the basis of the ratedcapacity of the welding current source 2, in particular the resonantcircuit 27. In the case of the frequency curve illustrated in FIG. 4, acharacteristic curve is plotted with a minimum resonance frequency 55and a maximum resonance frequency 56. The minimum resonance frequency 55switches in whenever a short circuit occurs between the welding wire 13and the workpiece 16 so that the ohmic resistance 53 becomes zero. Themaximum resonance frequency 56 switches in whenever the arc 15 isextinguished between the welding wire 13 and the workpiece 16 becausethe ohmic resistance 53 becomes infinite.

During operation of the welding current source 2, in other words duringa welding process, the resonance frequency of the resonant circuit 27may shift within these two frequencies as the loads vary, and, in orderto ensure safe operation, a bias point 57, schematically indicated onthe curves, is fixed on one side of the resonance frequency, inparticular above the resonance frequency, and this bias point 57 can beshifted to the required output along the schematically indicated curveat a constant resonance frequency when adjustments or controls areapplied by the control system 4. Consequently, different curves occurdue to the different output states during a welding process, lyingbetween the minimum resonance frequency 55 and the maximum resonancefrequency 56. To this end, the frequency f is plotted on the abscissa ofthe schematic diagram and the transfer function G on the ordinate, thetransfer function G specifying the power multiplier between outputvoltage and input voltage, i.e. if the transfer function G has a valueof 2, the output voltage will be twice the input voltage.

When using the resonant circuit 27 in the welding current source 2 inthis manner, however, care should be taken to ensure that when thewelding current source 2 is adjusted or controlled, the bias point 57 isalways kept on the same side of the resonance frequency, in other wordsabove or below the resonance frequency, because in the event of achangeover to the other side, the control or regulation principle isreversed, i.e. if the bias point 57 is set above the resonancefrequency, whenever there is a change in output, in other words wheneverthere is a change of resistance at the consumer and hence when theresonance frequency is changed, it must again lie above the newresonance frequency.

If, as indicated by broken lines, the bias point 57 is set above thisresonance frequency on the curve plotting the minimum resonancefrequency 55, for example, there will be a change in resonance frequencyand hence the curve, to the maximum resonance frequency 56 wheneverthere is a rapid change in output, in particular a rapid change ofresistance such as would occur on clearing a short circuit. As a result,the bias point 57 will then migrate to the new curve, as shown by thebroken line, that is to say below the resonance frequency, which willresult in a change in the control principle.

If, for example, the control system 4 is required to apply a reductionin output, a frequency increase will be needed if the bias point 57 isabove the resonance frequency so that the bias point 57 can be shiftedalong the falling curve, as is the case with the characteristic curve ofthe minimum resonance frequency 55.

Since however, as mentioned above, the bias point 57 was moved below theresonance frequency, as may be seen by the broken lines on the curve ofmaximum resonance frequency 56, the increase in the resonance frequencywill now be generate and apply an increase in output because it isshifted along the rising curve for the maximum resonance frequency 56,for example, in other words on the side below the resonance frequency,which can give rise to faulty functioning of the welding current source2 as well as faults in components. This particular procedure must bespecifically taken into account in particular when the characteristiccurve changes from a lower frequency to a higher frequency since in thereverse situation, in other words a change from a higher frequency to alower frequency, as indicted by broken lines, the bias point 57 isalways kept on the same side of the characteristic curve.

However, in order to prevent the bias point 57 from changing sides inthis manner, it is necessary to use a control and/or regulation methodof the type described below, which will ensure that the bias point 57 isalways kept on the fixed side of the characteristic curve accordingly,even when rapid changes occur in the resistance of the consumer as isthe case during a welding process, and preferably above the resonancefrequency. However, this is very difficult in applications involving awelding current source 2 because very rapid changes in output andchanges in resistance can occur, and the welding current sources with aresonant circuit known from the prior art usually have to be re-set inorder to be able to apply the corresponding control or correction.

To ensure that a change in the bias point 57 from one side to the otherside of the resonance frequency can not occur, a status variable of theresonant circuit 27, is applied as a means of controlling and regulatingthe welding current source 2, in particular the current curve or thevoltage curve in the resonant circuit 27, for example the resonancecurrent 58, as illustrated in FIGS. 5 to 7, in the form of an adjustmentor control variable. Instead of the resonance current 58, it wouldnaturally also be possible to apply the resonance voltage in theresonant circuit 27 for adjustment and control purposes, in which casethe characteristic curve is shifted by 90E. This ensures that wheneverthere is a change in frequency in the resonant circuit 27, the biaspoint 57 is moved or shifted accordingly, i.e. in the event of a changein resistance, in other words a change in load, the welding currentsource 2 is switched and adjusted at least to the resonance frequency orabove the resonance frequency and the bias point 57 is therefore notable to migrate to the other side of the resonance frequency and anyshift of the bias point 57 to the correct side of the currentcharacteristic curve can be operated by a frequency increase orfrequency reduction accordingly.

In order to be able to describe in more detail how the welding currentsource 2 is regulated and controlled and in particular how the bridgecircuit 28 and its switching elements 32 to 35 are activated, FIGS. 5 to7 illustrate different regulating and control sequences. For example,FIG. 5 illustrates a regulating and control sequence in the case of aconstant resonance frequency, in other words with the output mode of theconsumer unchanged, whilst FIG. 6 illustrates the situation in the eventof an increase in resonance frequency, such as would occur when a shortcircuit is broken or when increasing an arc length or extinguishing thearc 15, and FIG. 7 illustrates the situation in the event of a reductionin the resonance frequency such as would occur in the event of a shortcircuit or when shortening an arc length. The regulation and controlsequences illustrated occur unexpectedly in a welding current source 2and a corresponding adjustment or control has to be carried out everytime.

The control and/or regulation procedure is operated on the basis ofseveral parameters and FIGS. 5 to 7 plot a status variable, inparticular the resonance current 28 of the resonant circuit 27, a zerocrossover detection 59 operated by the control system 4 or measuringdevice 47, a ramp curve 60 with control variables alpha and phi, as wellas voltage curves 61 to 64 of the switching elements 32 to 35 and avoltage curve 65 of the bridge circuit 28. The individual curves in theindividual diagrams are synchronous in time.

For regulation and control purposes, several control states S1 to S4 forthe bridge circuit 28, in particular its switching elements 32 to 35,are stored in the control system 4, which are called up depending on theprevailing output conditions at the welding current source 2, in otherwords at the welding torch 10. The possible sequences for applying theindividual control states S1 to S4 are indicated by arrows in FIG. 3.

The stored control states S1 to S4 are set out and stored in the tablegiven below and the “on” state for a switching element 32 to 35 isactivated accordingly by the control system 4. The table below alsoshows other control states S5 and S6 , relating to another embodiment—asillustrated in FIGS. 8 and 9.

Switching elements Control states: 32 33 34 35 power supply S1 on offoff on positive drive phase S2 on off on off positive freewheel S3 offon on off negative drive phase S4 off on off on negative freewheel phaseS5 off on off off special operating mode S6 on off off off specialoperating mode

The specification for the power supply was arrived at on the basis thatit describes the power feed into the resonant circuit 27 of the weldingcurrent source 2 from the power source 29 via the bridge circuit 28, asillustrated by the voltage curve 65, i.e. during the positive drivephase and the negative drive phase of an intermediate direct currentcircuit of the welding current source 2, in other words from the powersource 29, current flows across the switching elements 32 to 35 into theresonant circuit 27 and hence to the consumer, in particular the weldingtorch 10, whereas during the positive or negative freewheeling phase, nopower supply or no flow of current takes place via the switchingelements 32 to 35 of the bridge circuit 28 from the intermediatecircuit, but a current circuit is maintained in the resonant circuit 27and the latter resonates independently.

This being the case, the current flow in the drive phases is generatedby activating switching elements 32 and 35 or 33 and 34 in pairs,whereas during the freewheeling phase, switching elements 32 and 34 or33 and 35 are activated and a common potential is therefore applied tothe switching elements 32 to 35 of the resonant circuit 27.

In principle, it should be pointed out that, because of the loss inoutput which occurs in the components during a switching procedure, theswitching elements 32 and 33, for example the IGBT transistors, areswitched, shortly before or after a current zero of the resonancecurrent 58 in the resonant circuit 27, to a specific control signalalpha—as per the ramp curve 60—whereas the switching elements 34 and 35,in other words the MOSFET transistors, are switched to a specificcontrol signal, in particular a phase angle phi, of the resonancecurrent 58 in the resonant circuit 27, i.e. the current flow in theresonant circuit 27 is applied as a control and regulating variable andthe welding current source 2 with the resonant circuit 27, in particularthe serial/parallel converter, is operated at or above the resonancefrequency, in other words at or above the characteristic frequency,without having to use or apply an external variable, as is the case withresonators known from the prior art. Naturally, it would be possible touse other transistors and the switching elements 32 and 33 could beprovided in the form of MOSFET transistors and the switching elements 34and 35 as IGBT transistors.

The control and regulating variables needed by the control system 4 tocontrol or regulate the bridge circuit 28 are the control signals alphaand phi and the zero crossing of a status variable, in particular theresonance current 58 or the resonance voltage, the control signal alphabeing responsible for activating switching elements 32 and 33 in theregion of the current zero and the control signal phi for activatingswitching elements 34 and 35 at a specific phase angle of the currentflow in the resonant circuit 27. The control signals alpha and phi aretherefore calculated and set by the control system 4 depending on therequired output so that a corresponding pulse width of the bridgecircuit 28 can be obtained by activating the switching elements 32 to 35whilst the control signal of the current zero is generated insynchronisation with the zero crossings of the resonance current 58.

Different methods may be used to generate a pulse width suitable foractivating the switching elements 32 to 35. For example, the controlsignals phi and alpha are converted to a ramp function, for example, asillustrated by the ramp curve 60 in FIGS. 5 to 7, and compared with aramp signal 66 and when the control signals phi and alpha intersect orcoincide with the ramp signal 66, the switching elements 32 to 35 areactivated accordingly. However, to enable the ramp signal 66 to besynchronised with the resonance frequency of the resonant circuit 27,the continuously and linearly rising ramp signal 66 is restartedwhenever the resonance current 58 crosses zero.

On this matter, it should be pointed out that the values of the controlsignals alpha and phi at maximum pulse width or pulse breadth, in otherwords at maximum output power, may be equal in size and the weldingcurrent source 2 is operated at the resonance frequency, so to speak,which means that the value of the control signal phi is smaller thanalpha at a lower output power, and either both control signals are setsimultaneously or the control signal phi is set before the controlsignal alpha. When operating the welding current source 2 at theresonance frequency, it is also possible for the values of the controlsignals phi and alpha to correspond to the pulse width of the resonancecurrent, i.e. the values of the control signals will match the maximumvalue of the ramp signal 66 to be attained, in which case the switchingelements 32 to 35 are activated and deactivated simultaneously andimmediately after every current zero, in accordance with the switchingand control timing. The pulse width is therefore determined by thedifference between the two control signals phi and alpha.

Naturally, it would also be possible for this comparison or the processof setting the timing with which the switching elements 32 to 35 areswitched on and off to be run in digital format, for example by means ofa counter, or on the basis of a simple computation run by the controlsystem 4.

In the embodiment illustrated as an example here, the ramp signal 66 isformed in such a way that it rises to a fixed value within the durationof a half-period of the resonance current 58, in other words between twocurrent zeros, as a result of which a control is applied to the bridgecircuit 28 by the control system 4 within the duration of a half-wave ora half-period of the resonance current 58, since the control signals phiand alpha are set during normal operation.

However, because of the different period duration, that is to saybecause of the different resonance frequencies of the resonant circuit27 due to differing output conditions, it may be that the duration of ahalf-wave and the duration of a half-period within which the ramp signal66 must rise to the set value will be changed because of the change inresonance frequency, i.e. whenever there is a change in output forexample, the resonance frequency of the welding current source 2changes, this change in output being due in particular to a change inresistance at the consumer because a short circuit has occurred, the arcis in operation or the arc is extinguished, which means, for example,that the duration of the period, in particular a half-period 67 of theresonance current 58 can be shortened or lengthened, so that the rampsignal 66 has not yet reached or has already exceeded the set value.

Consequently, in the case of an increase in resonance frequency, forexample, the ramp signal 66 can not reach the pre-set value but isalready interrupted and re-started at any instant 68, as may be seenform FIG. 6, or in the event of a reduction in the resonance frequency,the value has already been reached or exceeded and a current zero hasstill not occurred, as may be seen in FIG. 7 at the instant 68.

For example, it may be, as illustrated in FIG. 6, that the controlsignals phi and alpha for activating the switching elements 32 to 35 lieoutside the range, i.e. the ramp signal 66 is interrupted and re-startedbefore reaching the values of the control signals phi and alpha, so thatit will no longer be possible to activate the switching elements 32 to35 depending on the control signals phi and alpha because the currentflow or the resonance frequency has changed and the sinusoidal resonancecurrent 58 is therefore been changed from the positive half-wave to thenegative half-wave or vice versa before the control signals phi andalpha occur, even though the switching elements 32 to 35 are stillswitched for the preceding half-wave.

This state is detected in such a way and monitored in such a way by thecontrol system 4 that every current zero of the resonance current 58 isdetected by the control system 4 and the control system 4 runs a checkafter the onset or activation of the current zero to ascertain whetherthe control signals phi and alpha, which are compared with the rampsignal 66, have already been activated or not so that the control stateS1 to S4 to which the switching elements 32 to 33 must be switched canbe determined by the control system 4.

The switch between the individual control states S1 to S4 is effected insuch a way that when the welding current source 2 is operating in stablenormal operation without any change in frequency—as illustrated in FIG.5—the bridge circuit 28 is switched from the positive drivephase—control state S1—into the positive freewheeling phase—controlstate S2—and from it into the negative drive phase—control state S3—andthen into the negative freewheeling phase—control state S4. The switchmade from the negative freewheeling phase to the positive drive phasecloses the control circuit. This sequence is run by the control system 4if the bridge circuit 28 is stable, operating above the resonancefrequency as illustrated in FIG. 5, and hence the half-pulse width 67between the current zeros remains constant or approximately the same.

However, if it happens that the resonance current 58 crosses zero beforeone of the two control signals phi and alpha or between them, as is thecase at instant 68 in FIG. 6 for example, a special control process, inparticular a special operating mode, is initiated by the control system4 in order to synchronise with the new resonance frequency of theresonance current 58 and simultaneously prevent components from damage,in particular the switching elements 32 to 35, due to shutdown at aninadmissible current flow in the form of a change of potential. To thisend, the control system 4 switches form a drive phase—control state S1or S3—into the other drive phase—control S3 or S1—immediately thecurrent crosses zero.

Thereupon, the control system 4 runs a check to ascertain whether thetwo control signals phi and alpha have already been set or not, beforethe next current zero. If such is not the case, a switch is effectedinto the next drive phase, as may be seen in FIGS. 6 and 7. As a resultof switching from one drive phase into the next drive phase, the weldingcurrent source 2 is operated at the resonance frequency for a briefperiod when a current zero occurs, in order to prevent the bias pint 57from shifting to the other side, as described above. Operating at theresonance frequency also provides an opportunity to re-synchronise thebridge circuit 28 and the ramp signal 66 with the new resonancefrequency.

To enable a reduction in the resonance frequency to be likewisedetected, the control system 4 checks the value of the ramp signal 66whenever a current zero occurs and ascertains whether the value has beenreached or already exceeded so that a special operating mode can beinitiated again by the control system 4. Naturally, this monitoringsystem could also be used for a frequency increase since the controlsystem would merely have to check whether the control signals phi andalpha have already been set in the event of a current zero.

To enable the ramp signal 66 and hence the other control signals phi andalpha to be adapted to the new resonance frequency of theserial/parallel resonant circuit 27, this new period 69, in particularthe new half-period 67 within which the ramp signal 66 must reach theset value, is detected by the control system 4, which means that theperiod for the ramp signal 66 can be reduced or increased by a pre-setprocess, i.e. the control system 4 constantly detects the period 69between two current zeros, in other words the half-period 67 of theresonance current 58, and applies a change to the ramp signal 66accordingly in the event of a variance.

This being the case, it is possible for the ramp signal 66 to be unknownfor the duration of a half-period and when the next current zero occursthe ramp signal 66 is generated and adapted within its duration to thenew duration 69 so that it can in turn attain the prescribed valuewithin the new duration 69. Consequently, synchronisation to the newresonance frequency takes place within a half-period 67 of the resonancecurrent 58, ensuring that the control signals phi and alpha are re-setand activated if there is no further change in the resonance frequency,thereby enabling stable operation.

On switching from one drive phase into the other drive phase, theswitching elements 32 to 35 of the bridge circuit 28 are switcheddirectly to the current zero in accordance with the control states S1 toS4 described above. This is rendered possible because a still very lowresonance current 58 with a reversed sign prevails, in other words thechange from a positive half-wave into a negative half-wave or vice versadue to a very rapid control, which still enables the switching elements32 or 33 to be switched off and switched on without destroying thecomponents. The current level can be monitored for this purpose, forexample, so that when a fixed value is exceeded, the welding currentsource 2, in particular the bridge circuit 28, is briefly switched offin order to avoid causing a surge in the switching elements 32 to 35 atan inadmissible current flow, as would occur in the event of anunforeseen change from the positive to the negative half-wave or viceversa.

However, another option is to switch the switching elements 32 to 35into a special state, in particular special operating mode, on the basisof the control states S5 and S6 —illustrated in FIGS. 8 and 9—as will bedescribed in more detail below.

By switching the bridge circuit 28 from a drive phase into afreewheeling phase, in other words during normal operation and operationabove the resonance frequency, and by switching from a drive phasedirectly into another drive phase during special operating mode oroperation at the resonance frequency, the ramp signal 66 needed forcontrol purposes, in particular the duration 69 within which the rampsignal 66 must reach a pre-set value, can be adapted to the resonancefrequency, in particular to the duration 69 of the half-wave and ahalf-period 67 between two current zeros of the resonant circuit 27. Theduration for the ramp signal 66 may be changed using a number of methodsknown from the prior art, for example by adapting to the precedingduration or by a simple percentage increase or reduction, etc. Forcontrol purposes, the reduction or increase of the period 69 for theramp signal 69 is not crucial because in special operating mode, theswitching device 4 always switches from a drive phase into another drivephase and does not return to the normal switching cycle again until thecontrol signals phi and alpha are set prior to the next current zero.

However, the welding current source 2, in particular an inverter, can besynchronised with the new resonance frequency by means of the sequenceillustrated in FIG. 3 and thus transferred into a special operatingmode—based on the control states S5 and S6 illustrated in FIG. 8. Thiswill be described in more detail below.

In summary, therefore, it may be said that the control states S1 to S4and S1 to S6 for the switching elements 32 to 35 of the bridge circuit28 can be set by the control system 4 depending on the control signalsphi and alpha and the current zero in the resonant circuit 27 andapplied accordingly, whereby a switch can be effected from a drive phaseinto an other drive phase, etc., during operation above the resonancefrequency. The switching from one drive phase into the next drive phasecontinues until it becomes possible to operate above the resonancefrequency again, in other words until the current curve in the resonantcircuit 27 and the ramp signal 66 have been synchronised and operationof the welding current source 2 with the serial/parallel converter isreturned to a state above the resonance frequency and the controlsignals phi and alpha have been set again.

With regard to the control and/or regulating methods illustrated inFIGS. 5 to 7, it should be pointed out that when a switching element 32to 35 is activated, it is so in such a way that the serially connectedswitching elements 32 and 33 or 34 and 35 are switched in acomplementary manner, i.e. for example on deactivating the switchingelement 32 or 34, the switching element 33 or 35 is simultaneouslyactivated later, in particular after what is referred to in technicaljargon as a fixed delay-time. However, it would naturally also bepossible to switch the individual switching elements 32 to 35consecutively.

To ensure that the actual welding current for the consumer, inparticular the arc 15, can be incorporated in the control and regulationprocess, another measuring device 70 is provided at the output of thewelding current source 2 for detecting the output current and the outputvoltage. It is in turn connected via lines 71, 72 to the control system4 so that the output current can be regulated to the pre-set desiredvalue. It is necessary to incorporate the output current in the controland regulation process in order to set and compute the pulse width forthe bridge circuit 28 so that the bias point 57 can be shifted along theset characteristic curve in accordance with the requisite output—asillustrated in FIG. 4—by varying the pulse width, and it may thereforebe said that the bridge circuit 28, in particular the half-bridge orfull bridge, is controlled by a process of pulse width modulationcombined with a variable period duration or period of time.

The sequence of the method proposed by the invention as a means ofregulating a welding current source 2 with a resonant circuit 27 maytherefore proceed in the manner described below, for example.

The energy supplied by a power source is fed via the bridge circuit 28to the resonant circuit 27, in which a consumer is disposed. Theconsumer is usually an arc 15 of a welding process, which is suppliedwith voltage and current pulses during normal operation, generated bythe bridge circuit 20 by means of the switching elements 32 to 35 28.During normal operation, the bridge circuit 28 and its switchingelements 32 to 35 are controlled by the control system 4 in such a waythat a bias point 57 on the characteristic curve of the resonant circuit27 lies outside a resonance frequency. This normal operation occurs if amore or less constant resistance prevails at the consumer. If theresistance of the consumer changes, this will lead to a change inresonance frequency. In order to enable fault-free operation of theswitching elements 32 to 35 in a phase of variation of the resistance ofthe consumer, the latter are switched at least to the resonancefrequency of the resonant circuit. The switching elements 32 to 35 areswitched so that the bias point 57 is always on the same side during acontrol process, in particular on the falling or rising side of thecharacteristic curve of the resonant circuit 27, in other words isalways kept on the same side relative to the resonance frequency. Theside on which the bias point 57 lies relative to the resonance frequencyis fixed by the position of the bias point 57 on the characteristiccurve of the resonant circuit 27 at which the bias point 57 was locatedimmediately prior to the change in resistance at the consumer. Thisbasic process sequence applies to all embodiments of the presentapplication, the only difference residing in the nature of the controlstates and the switching duration of the switching elements 32 to 35 inaccordance with the control states S1 to S4 described with reference tothe embodiment illustrated in FIGS. 2 to 7 and the control states 1 to 6explained with reference to the embodiment illustrated in FIGS. 8 and 9.

FIGS. 8 and 9 illustrate another embodiment of the method of controllingand/or regulating the bridge circuit 28, for which the control states S5and S6 are used.

The individual switching elements 32 to 35 are again activated dependingon the output conditions at the output terminals 51, 52, and, duringstable operation, in other words above the resonance frequency, thewelding current source 2, in particular the switching elements 32 to 35of the bridge circuit 28, are switched from a drive phase S1 or S3 intoa freewheeling phase S2 or S4, as illustrated in FIGS. 2 to 7. However,if a change in output occurs, in particular a change in resistance atthe consumer, the resonance frequency of the resonant circuit 27 alsochanges, as described in detail above with reference to FIGS. 2 to 7.

In this embodiment, there is no longer a switch from one drive phase S1or S3 into another drive phase S3 or S1—as described with reference toFIGS. 2 to 7—and instead, a switch is made into a special operatingmode, in which the control states S5 and/or S6 are used, depending onthe control, state S1 to S4 which prevails in the bridge circuit 28 atthe time. As a result of this special operating mode, when a change inoutput occurs or an increase or reduction in the resonance frequency,the control system 4 switches the switching elements 32 to 35, from thedrive phase S1 or S3 into the special control state S5 or S6 asindicated in the table above, in which case the switching elements 34 or35 are deactivated and the associated switching elements 33 or 32 remainactive. As a result, the current flow via the bridge circuit 28 from thepower source 29 is actively interrupted and the welding current source2, in particular the inverter, can be adapted to and synchronised withthe new resonance frequency. By providing the freewheeling diodes 36 to39 and as a result of the integrated freewheeling diodes of the powertransistors, however, the circuit of the resonant circuit 27 ismaintained, so that the current zeros of the resonance current 58 canstill be evaluated by the control system 4 and hence the synchronisationprocess operated.

This can be seen with effect from the instant 68 in the embodimentillustrated in FIG. 9. At this instant 68, the control system 4 detectsthat the current zero, described above, was set before the controlsignals phi and alpha, which means that it is no longer possible toswitch into one of the next control states S1 to S4. The bridge circuit28 is now controlled by the control system 4 in such a way that a switchis made from the currently prevailing drive phase S1 into the controlstate S5—illustrated in FIG. 8—for which purpose the switching element32 is deactivated and the switching element 33 activated. Switchingelement 35 is simultaneously deactivated so that the current flow acrossthe bridge circuit 28 is actively interrupted but the flow of currentfor the resonant circuit 27 via the freewheeling diode 39 of switchingelement 35 continues to be maintained. As described above, the duration69 for the ramp signal 66 can now be changed in a whole range of ways inorder to synchronise with the new resonance frequency renderingoperation above the resonance frequency possible again.

In principle, it should be pointed out that as a result of theindependent resonance of the resonant circuit 27, including outside ofthe drive phases, a flow of current is available in the resonant circuit27 and the current zeros can be constantly evaluated by the controlsystem 4, which also means that it is possible to synchronise during thecontrol states S5 and S6 in the special operating mode. Consequently, asa result of the storing the control states S1 to S4 in a defined manner,they can be assigned to the current zeros, in particular thehalf-periods of the resonance current 58, so that a controlled rise to alevel above resonance frequency is possible in normal operation—inaccordance with the control states S1 and S4 of FIGS. 2 to 7 or S5 andS6 of FIGS. 8 and 9 on the basis of the control states S1 and S4. Theswitch to the special operating mode S5 and/or S6 is thereforeundertaken by the control system 4 in this embodiment by increasing orreducing the resonance frequency, the detection and monitoring systembeing as described above with reference to FIGS. 1 to 7.

When the embodiment is in the state illustrated, when the next currentzero occurs, a switch is made from the control state S5 to the controlstate S6 and then from there to the control state S4, and, oncesynchronisation has been completed in the disconnected control state S1,the next drive phase can be initiated. This being the case, it ispossible to switch backwards and forwards several times between thecontrol states S5 and S6 or, after the first control state S5, back intothe normal control circuit, or after the special operating mode has beencalled up for the first time—control state S5 or S6 —there willnecessarily be a forced switch to the next control state S6 or S5 beforereturning to normal operation. However, on leaving normal operation, itis necessary to revert to the correct control state S1 to S4 of thecontrol sequence in normal operation. This is necessary because thesinusoidal course of the resonance current 58 at a current curve with afalling potential and an incorrectly assigned control state S1 to S4 canlead to surging in the components.

Consequently, it is necessary for the control system 4 to be able toassign the control states S1 to S4 to the currently prevailinghalf-periods of the resonance current 58 on a constant basis so thatafter synchronisation, the bridge circuit 28 can be switched back tonormal operation at the correct point in time.

With the embodiments described above and illustrated in FIGS. 1 to 9, itis also possible to depart from the illustrated control states S1 to S6and run a special control process, which will be described below. Thisis necessary because when using a welding current source 2 with aresonant circuit 27, it must always be operated above or at theresonance frequency and if there are unexpected and strong changes inoutput, in particular changes in the resistance of the consumer, it ispossible that the synchronisation can not always be operated within apre-set and pre-adjustable period and the maximum admissible desiredvalues may be exceeded accordingly. If the synchronisation proceduretakes too long, a situation can arise in which the independent frequencyof the resonant circuit 27 adjusts and it is therefore no longerpossible to operate the welding current source 2 because the controlstates S1 to S6 can no longer be assigned to the half-periods of theresonance current 58 and the welding current source 2 therefore has tobe re-started and run back up again.

FIGS. 10 and 11 provide schematic illustrations of embodiments forrunning a special control process. FIG. 10 shows an output curve 73 ofthe welding current source 2, obtained using the resonant circuit 27 andcontrolling the control states S1 to S6 as well as the special controlprocess. FIG. 11, on the other hand, is a block diagram illustrating thepossible parameters for the special control process, which are fed intothe control system 4 and applied for subsequent processing.

This output curve 73 plots the output voltage U on the ordinate and thecurrent I on the abscissa, the output curve 74 plotted in broken linesbeing that which would apply using the prior art.

In the block diagram given in FIG. 11, a converter 75 is connected to alogic unit 76 and the measurement signals from the measuring devices 47and/or 70 are applied to an input 77 of the converter 75, therebyenabling the control signals phi—line 78—and alpha—line 79—to begenerated. A control line 80 is also shown, by means of which the logicunit 76 is able to reset the converter 75, in other words a RESET signalis transmitted. The block diagram given in FIG. 9 may be applied as anequivalent circuit diagram for the control system 4, i.e. the functionsillustrated will be run by the control system 4.

Several comparators 81 to 84 are also provided, the purpose of which isto compare the current and voltage values supplied by the measuringdevices 47 and/or 70 during operation, in other words the actual values,with corresponding stored desired values, so that a control can beapplied whenever the desired values are exceeded, thereby preventingdamage to the components due to too high current and/or voltage values.The purpose of the comparator 81 is to compare the resonant current “Ires” supplied by the measuring device 47 with a predetermined maximumpermissible current “I max”, and output a signal “I resmax” to the logicunit 76 via a line 85 whenever the desired current “I max” is exceeded.The other comparator 85 in turn compares the resonant current “I res”with the zero potential and a signal is transmitted via line 86 wheneverthe resonant current “IresO” crosses zero and forwarded to the logicunit 75.

The other comparator 83 compares the welding voltage “U” from themeasuring device 70 with a pre-set maximum permissible desired voltage“U max” and whenever this desired voltage “U max” is exceeded, a signal“U resmax” is transmitted to the logic unit 75 via a line 87.

The other comparator 84 is used to monitor the temperature “T” for acooling unit provided in the welding apparatus 1 with a maximum desiredvalue “T max”. Naturally, it would also be possible to use othermonitoring systems known from the prior art to ensure safe operation ofthe welding current source 2.

In principle, it should be pointed out that—as shown by the curve ofFIG. 10—the welding current source has a rated capacity to generate amaximum output current at a corresponding output voltage and istherefore capable of supplying a high voltage such as required forignition purposes. In the case of the output curve 74 based on the priorart, at a maximum output current of 140 A and an output voltage of 50 V,the welding current source must generate an output of 7 kW in order toin order ignite the arc 15.

By using the welding current source 2 with the resonant circuit 27proposed by the invention, a maximum output voltage of 90 V at apotential output current of 140 A is possible for igniting the arc 15, amean value of the illustrated curve being used as the basis for ratingthe capacity of the welding current source 2, which means the weldingcurrent source 2 would therefore able to achieve an output ofapproximately 5 kW, i.e. as a result of the course of the special courseof the output curve 73, a very high output voltage is available at a lowcurrent flow, and the high output voltage thereby ensures that a stablearc 15 can be generated at a low current flow and the arc 15 can beignited due to the high output voltage.

The special course of the output curve 73 is achieved due to the factthat a correspondingly high amount of energy is available in theresonant circuit 27, in other words in the inductor 44 and in thecapacitors 45, 46, which can be forwarded to the output for igniting andmaintaining the arc 15 and for triggering a short circuit, without thewelding current source 2 having to be rated to the specific capacity togenerate this output voltage and the possible output current.

With regard to the illustrated output curve 73, the current and voltagespecifications on the diagram are based on an embodiment of a weldingcurrent source 2, in which, as a result of changing the rated capacityof the resonant circuit 27 and the power component 3 accordingly, thevalues for the output curve 73 can be varied, i.e. the maximum outputvoltage and the maximum output current can be varied on the basis of therating and the settings for the maximum possible values “I max and Umax”.

Consequently, if using a welding current source known from the priorart, it would be possible to obtain a rating of 90 V for the maximumoutput voltage and a maximum output current of 140 V, in which case thiswelding current source would have to supply an output of 12.6 kW asindicated by the output curve 74 shown in broken lines. For a standardwelding process, such as can be operated by the welding current source 2proposed by the invention with an output of 5 kW, this welding currentsource would therefore have considerably overrated capacity and wouldsimultaneously mean having to use a much bigger and heavier weldingcurrent source.

The schematically illustrated output curve 73 shown in FIG. 10 for thewelding current source 2 proposed by the invention is generated by thefact that the components and the power component 3 can be rated toprovide a corresponding energy supply, whereby the special course of theplotted output curve 73 is generated as a result of the influence of theresonant circuit 27, i.e. the output curve 73 can basically correspondto a rectangular diagram of the broken line of a characteristic curve asplotted on the basis of the prior art and the energy available in theresonant circuit 27 can be used to change the output curve 73 inaccordance with the schematic diagram.

For example, starting from a current value 88 of approximately 110 A, itis possible to supply an output voltage of approximately 25 V for thewelding process. This is necessary because, as indicated by anotherstandard characteristic curve 89, an output voltage of approximately 25V is necessary for a welding process with an output current of thistype. Starting from a rated capacity of the welding current source 2 inthe range illustrated by the current value 88, the welding currentsource 2 is able to deliver a maximum output current 90 of 140 A, forexample, with a lower output voltage 91 of approximately 15 V, as aresult of which more power can be supplied for resolving a shortcircuit, making continuous operation of the welding apparatus at acurrent corresponding to the current value 88 possible. By reducing thecurrent, the welding current source 2 increases the voltage and areduction to approximately 60 A will give rise to an increase in thevoltage to 40 V, for example. From this point, an exponential voltageincrease occurs, and the output voltage—as illustrated in FIG. 11—ismonitored so that when the maximum permissible desired voltage U max isexceeded, corresponding to a voltage value 92, the control system 4initiates the special control process described below, thereby limitingthe voltage. If this were not the case, the voltage would riseinfinitely as indicated by the broken line, in other words would belimited by the power losses of the components, which would damage thecomponents. As a result of the special control process initiated by thecontrol system 4, the voltage is controlled and limited to a predefinedvalue.

The advantage of this type of output curve 73 resides in the fact that,with a low current flow, a corresponding high output voltage isavailable for maintaining the arc 15, so that the power component andthe welding current source 2 can be kept to a low capacity rating sincethe extra energy that is needed can be supplied from the resonantcircuit 27.

Since the resonance frequency of the serial and/or parallel resonatingcircuit, in particular the resonant circuit 27, in the welding currentsource 2 is set depending on the output state at the consumer, whenthere is a significant change in output, in particular significantchanges in resistance, a situation may arise in which thesynchronisation can not be completed in a given time and the naturallyresonating resonant circuit 27 will automatically be terminated. Thissituation can happen because even during a freewheeling phase or duringthe special operating mode in which no energy is supplied by the powersource 29, energy is discharged to the consumer causing energy to belost by the natural component losses on the current source, which meansthat the welding current source 2 has to be re-started and run up again.

To prevent this from happening, the control system 4 can run the controland/or regulating process with the special operating mode describedabove by switching from a drive phase into another drive phase or intothe special operating mode S5 or S6 to run a special control process. Tothis end, whenever a parameter is exceeded, in particular the resonancecurrent 58 or the welding voltage, it is possible to call up and run aspecial control process by means of a pre-set desired value.

In effect, if there is a change in output, the control system 4 switchesthe bridge circuit 28 to the special state S5 and S6 , in which case thecontrol system 4 will monitor how often a switch is made from a specialstate S5 or S6 into another special state S6 or S5. The number of timesswitching may be operated backwards and forwards between the specialstates S5 and S6 is stored in the control system 4 and is preferablyfour times. If switching backwards and forwards between the specialstates S5 and S6 has happened too often, the naturally resonatingresonant circuit 27, in particular the resonance current 58 and/or theresonance voltage, would be drained because of the component losses andit would no longer be possible to run a welding process because thepower source 29 would not be able to supply the resonant circuit 27 withenergy.

If the special operating mode, in other words call-up of the controlstates S5 or S6 , exceeds the pre-set switching value—preferably fourtimes—the control system 4 switches the bridge circuit 28 into thespecial control process, as a result of which the switching elements 32to 35 are activated in the form of the drive phases. This being thecase, however, the pulse width is reduced to a minimum so that the powersource 29 can generate a low energy feed, thereby maintaining theresonance of the resonant circuit 27. The supply of energy my thereforebe applied over several periods and the duration of the period or thenumber of periods is stored in the memory system 4 so that when thisspecial control process has been completed, the system can be returnedto the control state S5 or S6 which prevailed before and monitoringre-initiated to ascertain whether it will now be possible to run asynchronisation. A return to the previously prevailing control state S1to S6 is possible at any time because the control states S1 to S6 can becorrelated by the control system 4 to the half-periods of the resonancecurrent 58, even during the various special situations, which means thatit will always be possible to return to a specific control state S1 toS6 at any time.

As a result of the resonance of the resonant circuit 27, the voltage isable to rise above a pre-set maximum voltage value and/or currentvalue—illustrated in FIGS. 10 and 11—so that when a situation of thistype occurs, the control system 4 an in turn run a special controlprocess. This monitoring process is briefly illustrated in FIG. 11 bythe described and illustrated parameters that are monitored. If such asituation arises, the pulse width for the bridge circuit 28 is firstlyreduced to a minimum. Simultaneously, a monitoring process ascertainswhether, after the resonance current 58 has crossed zero once or more,the signal “I resmax and/or U max” has fallen below the correspondingdesired value or not. The process of counting the number of current zerocrossings can be pre-set for the purpose of reducing the parametersbelow the desired values and stored in the control system 4. Thisprocedure can be run several times but every time a specific number ofsuch control attempts has been exceeded, the control system 4deactivates the bridge circuit 28, i.e. all switching elements 32 to 35are deactivated, so that the resonance current 58 and the resonancevoltage in the resonant circuit 27 are able to compensate for thecomponent losses, thereby enabling the welding current source 2 to bere-started and run back up again.

A control procedure of this type is illustrated in the diagram of FIG.10. In this case, as the output current drops, the voltage, inparticular the voltage value 92, of the output curve 73 rises above apre-settable voltage desired value “U max”, corresponding to the voltagevalue 92. If a special control process were not run in this situation,the voltage would continue to rise as indicated by the broken line. As aresult of this high voltage, components, in particular diodes and powertransistors, cold be damaged and it would be necessary to usehigher-capacity components in the welding current source 2 thannecessary. Consequently, if the voltage is at the desired value and hasthus reached the voltage value 92, the pulse width can be reduced sothat less energy is now supplied and the voltage is reduced again due tothe component losses and/or the energy supplied to the consumer. Thecontrol system then switches back to normal operation, in other words toone of the control states S1 to S4.

It may be, however, that the desired value is exceeded, as illustratedby the voltage value 93 on the output curve, for example, causing thepulse width to be reduced to a minimum over a pre-settable number ofhalf or full periods. This special control process can continue to berun until the resonant circuit 27 no longer contains any energy or,having reached a predetermined number of these control attempts, thebridge circuit 28 is deactivated by the control system 4 so that anyremaining energy in the resonant circuit 27 is run down. This is what ishappening with effect from the instant 94, at which the energy decreasesexponentially, i.e. the voltage increases exponentially and the currentdrops constantly. What this also means is that a very high voltage isavailable at the start of a welding process and for re-igniting the arc15. It is also possible to stored different voltage desired values fordifferent current values, enabling an exponential curve of this type tobe generated.

Consequently, it may be said that whenever a predefined and adjustabledesired value is exceeded, the control system 4 runs a special controlprocess, for which purpose the pulse width for the bridge circuit 28 isreduced to a minimum and, after the resonance current 58 has crossedzero once or more, the bridge circuit 28 is deactivated. To this end, atleast the output voltage at the consumer, in other words at the outputterminals 51 and 52, and the resonance current 58 are monitored andcompared with a desired value.

In summary, it may be said that if using the welding current source 2with the resonant circuit 27 in the form of a serial/parallel converter,the described method is such that in order to control the bridge circuit28, in particular half or full bridge, several fixed, pre-set controlstates S1 to S6 for the switching elements 32 to 35 of the bridgecircuit 28 are stored for the purpose of controlling the bridge circuit28, and; in a sequence controlled by the control system 4 during normaloperation, in other words above or below the resonance frequency, astatus variable of the resonant circuit 27, in particular the resonancecurrent 58 or the resonance voltage of the resonant circuit 27, thebridge circuit 28 is controlled on the basis of the control states S1 toS4 successively, so that whenever a change in output occurs, inparticular a change in resistance, at the consumer, a special operatingmode is run by the control system 4 with the bridge circuit 28 in or atthe natural frequency of the resonant circuit 27 corresponding to thecontrol states stored for the special operating mode, in particular thecontrol states S1 or S4 and S5 or S6 , whereby, in the individualoperating modes, in particular during normal operation, in specialoperating mode and/or during the special control process, the individualcontrol states S1 to S4 for normal operation are assigned to theindependently resonating resonant circuit 27, in particular the currentzeros of the resonance current 58 or the resonance voltage, by thecontrol system 4 and the control states S1 to S4 are therefore dependenton the status variable of the resonant circuit 27, in particular thecurrent zeros of the resonance current 58 or the resonance voltage.

Consequently, the welding current source 2 can be run in severaloperating modes, in particular a normal operating mode, a specialoperating mode and a special control process, so that the control system4 is able to regulate the welding current source 2, in particular thebridge circuit 28, in such a way that, in the event of a change in thecharacteristic curve of the resonant circuit 27—as illustrated in FIG.4—the bias point 57 is always kept on the same side, in particular onthe falling or rising characteristic curve of the resonant circuit 27.

With a welding current source 2 having a resonant circuit 27 of thistype, it is also possible to use a current converter 95, in particular atransformer, in which case the energy supplied by the power source 29can be converted. If provided, the current converter 95 may be arrangedbetween the bridge circuit 28, in other words the resonant circuit 27,and the rectifier 50, as illustrated in FIG. 12. However, it would alsobe possible for a current converter 95 of this type to be arranged inthe power source 29 itself.

Finally, it should be pointed out that the individual parts andcomponents or groups of components described with reference to theembodiments are illustrated in a simplified, schematic format.Furthermore, individual parts of the combinations of features describedabove or features of the individual embodiments described as examplesmay be used in combination with other individual features from otherembodiments and may thus be construed as independent solutions proposedby the invention

Above all, subject matter relating to the individual embodimentsillustrated in FIGS. 1, 2, 3, 4, 5, 6, 7; 8, 9; 10, 11; 12 can beconstrued as independent solutions proposed by the invention. The tasksand solutions can be found in the detailed descriptions relating tothese drawings.

List of reference numbers 1 Welding apparatus 31 Line 2 Welding currentsource 32 Switching element 3 Power component 33 Switching element 4Control system 34 Switching element 5 Switching element 35 Switchingelement 6 Control valve 36 Freewheeling diode 7 Supply line 37Freewheeling diode 8 Gas 38 Freewheeling diode 9 Gas storage 39Freewheeling diode 10 Welding torch 40 Control line 11 Wire feed device41 Control line 12 Supply line 42 Control line 13 Welding wire 43Control line 14 Supply reel 44 Inductor 15 Arc 45 Capacitor 16 Workpiece46 Capacitor 17 Supply line 47 Measuring device 18 Supply line 48 Line19 Cooling circuit 49 Line 20 Flow indicator 50 Rectifier 21 Watercontainer 51 Output terminal 22 Input and/or output device 52 Outputterminal 23 Hose pack 53 Resistance 24 Connector device 54 Lineinductance 25 Tension relieving device 55 Minimum resonance frequency 26Housing 56 Minimum resonance frequency 27 Resonant circuit 57 Operatingpoint 28 Bridge circuit 58 Resonance current 29 Power source 59 Zerocrossing detection 30 Line 60 Ramp curve 61 Voltage curve 91 Outputvoltage 62 Voltage curve 92 Voltage value 63 Voltage curve 93 Voltagevalue 64 Voltage curve 94 Instant 65 Voltage curve 95 Current converter66 Ramp signal 67 Period 68 Instant 69 Duration 70 Measuring device 71Line 72 Line 73 Output curve 74 Output curve 75 Converter 76 Logic unit77 Input 78 Line 79 Line 80 Control line 81 Comparator 82 Comparator 83Comparator 84 Comparator 85 Line 86 Line 87 Line 88 Current value 89Standard characteristic curve 90 Output current

1. Method of controlling a welding current source (1) having a resonantcircuit (27), provided in the form of a serial/parallel converter,whereby a bridge circuit (28), comprising individual switching elements(32-35), is controlled by a control system (4) and a consumer, inparticular a welding process, is supplied with energy, in particularwith voltage and current pulses from a power source (29), controls thebridge circuit (28), whereby the control system (4) controls the bridgecircuit (28) during normal operation in such a way that the bridgecircuit (28) is switched by the control system (4) consecutively on thebasis of the control states (S1-S4) stored in the control system (4) sothat the bias point (57) on a character curve of the resonant circuit(27) lies outside a resonance frequency and whenever a change inresistance occurs at the consumer, the control system (4) operates withthe bridge circuit (28) is operated at the natural frequency of theresonant circuit (27) on the basis of the control states and sequencestored for the special operating mode, characterised in the controlstates (S1-S4) fixed for normal operation of the bridge circuit (28)provided as a full bridge are a positive drive phase—control state(S1)-, a positive freewheeling phase—control state (S2) —, a negativedrive phase—control state (S3)—and a negative freewheeling phase—controlstate (S4)—and, in the special operating mode, the bridge circuit (28)is switched from a drive phase—control state (S1 or S3)—preferablyconsecutively, into one of several alternative control states (S5 or S6)in which the switching elements (34; 35) of one bridge branch aredeactivated and the switching elements (33; 32) of the other bridgebranch remain activated and the control system (4) monitors how often aswitch is made from one special mode (S5; S6) to the other special mode(S6; S5).
 2. Method as claimed in claim 1, characterised in that asequence of control states (S1-S4) of the switching elements (32-35) ofthe bridge circuit (28) is stored in the control system (4).
 3. Methodas claimed in claim 1, characterised in that the control states (S1-S4)are activated during a constant state in the conditions at the consumer,on the basis of a time difference between two immediately consecutivecurrent zeros of a status variable of the resonant circuit (27). 4.Method as claimed in claim 1, characterised in that the individualcontrol system (S1-S4) for normal operation, are derived from and/orassigned to a current zero of a status variable of the independentlyresonating circuit (27) in particular by the control system (4). 5.Method as claimed in claim 1, characterised in that the control system(4) continuously and repeatedly switches between the individual controlstates (S1-S4) when the welding current source (2) is running in normaloperation.
 6. Method as claimed in claim 1, characterised in that in thespecial operating mode, the control system (4) switches the bridgecircuit (28) directly from a drive phase—control state (S1 or S3)—intothe other drive phase—control state (S3 or S1) whenever the current zerooccurs.
 7. Method as claimed in claim 1, characterised in that in thebridge circuit (28), in particular the half or full bridge, iscontrolled by a process of pulse-width modulation combined with avariable period duration, with control signals phi and alpha.
 8. Methodas claimed in claim 1, characterised in that, depending on the controlsignals phi and alpha and the current zero of a status variable of theresonant circuit (27), in particular the current zero in the resonantcircuit (27), the control system (4) sets the control states (S1-S6 )for the switching elements (32 to 35) of the bridge circuit (28) andapplies these accordingly.
 9. Method as claimed in claim 1,characterised in that the switching elements (32 and 33), in particularIGBT transistors, are switched to a control signal alpha shortly beforeor after a current zero of the resonant circuit (27) and the switchingelements (34 and 35), in particular MOSFET transistors, are switched toa control signal phi, in particular a phase angle phi of the current inthe resonant circuit (27).
 10. Method as claimed in claim 1,characterised in that the control and regulating variables for thecontrol system (4) for controlling or regulating the bridge circuit (28)are generated by the control signals alpha and phi and by a statusvariable of the resonant circuit (27) crossing zero, in particular theresonance current (58) or the resonance voltage.
 11. Method as claimedin claim 1, characterised in that during the positive drivephase—control state (S1)—and the negative drive phase—control state(S3)—a flow of current passes from the intermediate circuit of thewelding current source (2), in other words from the power source (29),via the switching elements (32 to 35) into the resonant circuit (27) andhence to the consumer, in particular to the welding torch (10), whereasduring the positive or negative freewheeling phase control state (S2;S4)—no power supply and no flow of current passes via the switchingelements (32-35) of the bridge circuit (28) from an intermediate circuitinto the resonant circuit (27) but a flow of current is maintained inthe resonant circuit (27).
 12. Method as claimed in claim 1,characterised in that the flow of current in the drive phases isgenerated by activating the switching elements (32 and 35; 33 and 34) inpairs, whereas the switching elements (32 and 34; 33 and 35) areactivated during the freewheeling phase.
 13. Method as claimed in claim1, characterised in that the control system (4) constantly assigns thecontrol states (S1-S4) to the half periods of the resonance currentprevailing at any time so that when the bridge circuit (28) has beensynchronised with the correct timing, a switch is made back to normaloperation.
 14. Method as claimed in claim 1, characterised in thatseveral comparators (81-84) are connected to the control system (4) anda logic unit (76) and the comparators (81-84) compare the actuallyprevailing current and voltage values, in particular the actual values,delivered by a measuring device (47 and/or 70) with corresponding storeddesired values, a control being run by the control system (4), inparticular a special control process, if the desired values areexceeded.
 15. Method as claimed in claim 14, characterised in that thecomparator (81) compares the resonance current “I res” supplied by themeasuring device (47) with a pre-set maximum permissible desired current“I max” and a signal “I resmax” is output to the logic unit (76) via aline (85) if the desired current “I max” is exceeded.
 16. Method asclaimed in claim 15, characterised in that the control system (4)monitors to see whether, after several current zeros have occurred inthe resonant circuit, the signals “I resmax” and/or “U max” have fallenbelow the desired value “I max” and/or “U max”.
 17. Method as claimed inclaim 14, characterised in that the comparator (83) compares the weldingvoltage “U” of the measuring device (70) with a pre-set maximumpermissible desired voltage “U max” and a signal is output to the logicunit (76) via a line (87) if this desired voltage “U max” is exceeded.